There has recently been an increasing demand for an improvement in sound quality by picking up sound waves beyond the audio frequency range. According to “Hairezoryushon ohdio (saundo) no torikumi (efforts for high-resolution audio (sound))” released by the Japan Audio Society, a general incorporated association, on Jun. 12, 2014, a high-resolution device is defined as a device that has a recording microphone capable of performance in the high-frequency range of 40 kHz or more.
A hollow cylindrical condenser microphone having a diaphragm at one end generally suffers from diffraction effects resulting from its shape and thus cannot achieve such a performance unless the diameter of the hollow cylinder is made small. Measurement condenser microphones can pick up sound waves up to 100 kHz but have a ¼-inch diameter.
Such a small diameter offers a small effective capacitance between a diaphragm and a fixed electrode, which makes it difficult to ensure a signal-noise ratio (S/N ratio) required for picking up instrument sounds.
The invention disclosed in Patent Literature 1 (Japanese Patent Laid-Open No. 2004-282449) involves appropriate designs related to diffraction effects at the end of the hollow cylinder and the mechanical resonance frequency of a non-directional condenser microphone, so that the microphone ensures a signal-noise ratio required for picking up instrument sounds and picks up sound waves up to 100 kHz.
However, as shown in the graph of FIG. 6, the sound pressure detected at the end of the hollow cylinder is frequency-dependent on the diameter and wavelength of the microphone. Accordingly, increasing the diameter of a microphone for higher sensitivity decreases the frequency range it can pick up. It should be noted that the graph of FIG. 6 is cited from Non Patent Literature 1: FIG. 5-4 “Ento niyoru onpa no kaisekikouka (diffraction effects from sound waves produced by a hollow cylinder)” (Muller, Black, and Davis) in the article “Hosogijutsusha notameno maikurohon koza (5) keijo ya okisa ga oyobosu eikyo (learning of microphones for broadcasting engineers (5) Effects from shape and size)” by Jinichiro Nakamura, on Hosogijutsu in the October 1981 issue.
Patent Literature 2 (Japanese Patent Laid-Open No. 2007-36525) proposes a condenser microphone unit in which a fixed electrode is a solid cylindrical electrode and a hollow cylindrical diaphragm is engaged with the solid cylindrical electrode with a predetermined gap therebetween, thereby increasing the effective area and sensitivity of the capacitor element.
However, like a commonly-used condenser microphone, the condenser microphone unit according to Patent Literature 2 has openings on a bottom (an end facing a sound source) of a bottomed hollow cylindrical unit case in order to let sound waves in, thus receiving sound waves from the end surface intersecting the direction of the axis of the fixed electrode, i.e., the solid cylindrical electrode, and driving the hollow cylindrical diaphragm. This results in frequency-dependence occurring with sound waves beyond the audio frequency range.
Accordingly, an object of the present invention is to provide a high-resolution non-directional condenser microphone unit that has a capacitor element with a large effective area and a high signal-noise ratio but does not have frequency-dependence occurring with sound waves beyond the audio frequency range.